A width of a pattern such as a wiring to be formed on a semiconductor substrate is recently becoming narrower in order to meet the requirements of high speed and high density of semiconductor devices. Miniaturization of patterns is realized by shortening the wavelength of exposure light used in photolithography. Presently, the pattern rule of semiconductor devices is at the level of 100 nm or narrower. This size is shorter than the wavelength of exposure light. For example, ArF excimer laser used as an exposure light source has a wavelength of 193 nm.
Since a width of the pattern rule is shorter than the wavelength of exposure light, influence of the optical proximity effect by diffraction and the like cannot be neglected, and pattern deformation by the optical proximity effect is now conspicuous. There occurs a phenomenon that even if the line widths of patterns on a mask are the same, line widths transferred to a wafer are different depending upon a pattern distribution density.
The following patent Document 1 discloses the technique of correcting a variation in the line widths caused by a pattern distribution density. Patterns on a mask disclosed in patent Document 1 will be described.
In an area where light shielding patterns are disposed densely, a slit is formed in each light shielding pattern. In an area where light shielding patterns are disposed coarsely, a slit is not formed in each light shielding pattern. Since light is transmitted through the slit, optical densities in the dense and coarse light shielding areas can be made uniform. By making uniform the optical densities, it becomes possible to prevent patterns from being deformed by the optical proximity effect.
The shape of an end portion of a pattern is likely to be deformed by the optical proximity effect. If a line width in particular becomes narrow, it is conspicuous that opposite ends of a straight line pattern retreat and the straight line becomes short. This phenomenon is called “shortening”. A correction method called an optical proximity correction (OPC) has been proposed as a method of correcting a shape difference to be caused by the optical proximity effect between a mask pattern and a transferred pattern. The OPC method corrects a change in the size and shape of a transferred pattern, by partially broadening the mask pattern in advance toward a direction opposite to a direction of the pattern deformation when the mask pattern is transferred to the wafer, or by disposing a dummy pattern.
The following patent Documents 2 and 3 disclose the techniques of suppressing shortening by utilizing the OPC. For example, a pattern is designed to make end portions of an elongated mask pattern broader than a central portion of the pattern. This broadened portion is called a hammer head. By forming the hammer head to the opposite ends of an elongated mask pattern, shortening can be suppressed.
Patent Document 1: JP-A-2005-10635
Patent Document 2: JP-A-2004-302263
Patent Document 3: Specification of JP-2004-196963 (JP-A-2006-19577)